The present invention relates to metallic substrates having coated thereon a multiple layer coating. More particularly, the present invention relates to such metallic substrates having a multiple layer coating of polymers desirably selected to provide adhesion between the metallic substrate and a subsequently applied layer of a vinyl chloride polymer. In one embodiment, the invention is further directed to an electrical cable, particularly a communications cable, comprising the multi-layer coated metallic substrate in the form of a sheath system, and, optionally, further comprising a water absorbing composition therein.
Polymers of vinyl chloride such as polyvinyl chloride homopolymers or copolymers of vinyl chloride and other copolymerizable monomers such as methyl methacrylate are desirably employed in the preparation of insulating jacketing resins for use in communications cables and other electrical wiring due to the inherent resistance to burning, ready availability, and low cost of such polymers. Suitable resins additionally comprise large percentages of known plasticizing compounds in order to impart desired flexibility to such resins. Because of the large amount of such plasticizing compounds vinyl chloride polymer jacketing resins are particularly difficult to bond to other substrates, especially metallic substrates.
Communications cables desirably include in their construction a sheath structure normally provided in the form of a thin metallic strip which is folded circumferentially around centrally disposed conductors and overlapped or otherwise suitably joined to form a substantially cylindrical structure. Such sheath systems may further be corrugated to provide greater flexibility. The sheath system provides mechanical integrity to the resulting cable, as well as electromagnetic shielding and physical protection to the inner wires and bonding to ground for protection from lightning strikes. Suitable metals for such sheath systems include aluminum, copper, steel, lead, stainless steel, and coated varieties of stainless steel such as copper or chrome coated stainless steel.
It is previously known in the art to prepare coated metal substrates, particularly strips coated with one or more polymeric layers to provide a sheath system for use in cable constructions. Coatings of copolymers of ethylene and an ethylenically unsaturated carboxylic acid such as acrylic acid are disclosed in U.S. Pat. No. 3,681,515. Additional polymers suitable for coating metals comprising high molecular weight uniformly random copolymers of ethylene and acrylic or methacrylic acids have been disclosed in U.S. Pat. No. 4,351,931.
Multiple polymer coatings for use in bonded sheath containing cables are disclosed in U.S. Pat. No. 4,487,641. In this reference, coatings comprising a first adhesive material which is bondable to the metallic layer and a second adhesive material which is bondable to the extruded jacketing resin are employed, See Col. 6, lines 53-65. A similar structure is disclosed in U.S. Pat. No. 4,449,014.
Copolymers of ethylene and carbon monoxide and, optionally, additional copolymerizable comonomers such as vinyl acetate are previously known in the art. In PCT Application No. 85 1294 such olefin polymers are disclosed for use as heatable high frequency electro-magnetic radiation receptors. Adhesion of such polymers to polyvinyl chloride and polyvinylidene chloride polymers is disclosed.
In Japanese Kokai No. 57-165,427, copolymers of ethylene, carbon monoxide and vinyl acetate are disclosed which are useful adhesives for laminating polyvinyl chloride substrates including substrates which are highly plasticized, for example, with dioctyl pththalate.
Thus, it is previously known in the art to prepare two layered adhesive laminates useful for adhesion to metals on the one hand and polyolefin jacketing resins on the other hand. In addition, it is previously known that copolymers of ethylene and carbon monoxides may be adhesively joined to polyvinyl chloride resins. However, a problem still exists in the art in obtaining adhesion between the single or multiple layer adhesive laminates usefully adhered to metal substrates and the ethylene carbon monoxide containing copolymers usefully adhered to vinyl chloride polymer resins, and in particular, obtaining adhesion in such a structure to such vinyl chloride polymer resins that additionally comprise large quantities of a plasticizer. Accordingly, there remains a difficulty in the art in obtaining a cable construction comprising a metallic sheath in combination with a highly plasticized polyvinyl chloride jacketing resin.
An additional problem encountered in the field of communications cables is the need or requirement that the cable jacketing layer be easily removable for installation of grounding connectors yet retain sufficient adhesion to the underlying shielding tape assembly that abrasion or stretching forces encountered in shipping and installing not damage the cable structure causing delamination of the polyvinyl chloride jacketing resin.
Finally, due to exposure of communications cables to moist environments and even to standing water it is desirable to provide a communications cable which provides improved resistance to water permeation and longitudinal water travel within the cable such as might occur should the jacketing and/or the protective sheath assembly become abraided or severed thereby exposing the inner conductor assembly to water optionally at an elevated hydrodynamic pressure.